|MadSci Network: Biochemistry|
Microwaving makes bread chewy and/or hard (i.e. stale) because it turns water in the bread to water vapor. The water vapor leaves the bread reducing it's water content.
Here is a link to a great bread staling experiment:
Here is a link to another madsci posting on a related topic:
Try the links in the MadSci Library for more information on Biochemistry.